Comparison in Surface Modifications of Sintered and Single Crystal Ceramic Targets After Laser Ablation
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COMPARISON IN SURFACE MODIFICATIONS OF SINTERED AND SINGLE CRYSTAL CERAMIC TARGETS AFTER LASER ABLATION
K. H. YOUNGa) Superconductor Technologies Inc., 460-F Ward Drive, Santa Barbara, CA 93111
a)present address: Energy Conversion Devices, Inc., 1675 West Maple Road, Troy, MI 48084
ABSTRACT A systematic study of the surface morphology and chemical composition of several ceramic targets after pulsed laser ablation has been conducted using a scanning electron microscope with x-ray energy dispersive spectroscopy. We have examined single crystal LaAIO3 and NdGaO 3 disks, as well as powder sintered LaAIO3 and SrTiO 3 discs. These targets were rotated during laser ablation and concentric trenches were formed with distinctive surface morphologies due to different doses of laser energy. We observed varied light-solid surface interactions on the targets prepared by different methods. The sintered targets exhibited surface explosions after long period laser ablation, presumably due to sub-surface overheating [1]. The single crystal targets have smoother surface morphology and the resulting films have much fewer particles on the surface. As such, they are preferred for laser ablation applications. INTRODUCTION Laser ablation has been used widely in depositing metal, semiconductor, dielectric, and superconductor thin films. One of its major advantages over other growth methods is the stoichiometric transfer of the target material onto the substrate, especially for multi-component systems, such as high temperature superconductors. Recent studies on sintered YBa2Cu3Ox (YBCO) targets revealed that the target surface undergoes alterations both in chemical composition and in surface morphology [2,3]. The surface is filled with 'finger-like' and 'onion-like' features, and becomes Y-rich and Cu-poor after long periods of laser irradiation [3]. These evolutions consequently affect the film composition and growth rate. Therefore, it is very important to know the best way to prepare targets which will have the least evolution in the surface composition and morphology. In this study, we examined carefully the surfaces of four simple perovskite oxide targets (two sintered and two single crystals) after about 200 to 1000 laser shots per site. EXPERIMENTAL The single crystal targets (1 inch dia. and 0.25 inch thick) were grown by the Czochralski technique [4] and supplied by Applied Technologies Enterprises (LaAIO3, Irmo, South Carolina) and Litton Airtron (NdGaO 3 , Charlotte, North Carolina). The stoichiometrically sintered ceramic targets were fabricated by Superconductive Components Inc. (LaAIO 3 , Columbus, Ohio) and Kurt J. Lesker Company (SrTiO3, Pittsburgh, Pennsylvania). A Siemens XPS-2020 XeCI excimer laser was used with a pulse-to-pulse stability within 1%, repetition rate of 5 Hz, and energy density of about 5 Joule/cm 2 . The laser beam incident angle was 45* from the normal to the target surface. The distance between target and substrate was 4.5 cm. The targets were rotated during laser irradiation and formed concentric rin
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